Fire and smoke spread through the entire complex in the 3900-block of Belmont in Downers Grove.
Firefighters from nearly two dozen fire companies spent hours putting out the fast-moving blaze, which was eventually escalated to a three-alarm blaze.
Emergency crews were able to evacuate all 33 tenants without injury. Investigators were still trying to determine the cause of the fire, including trying to determine if a lightning strike sparked the flames.
Flames erupted just after midnight, and residents smelled smoke and called for help. When firefighters arrived, they found the flames had already engulfed most of the 16-unit building.
After emergency crews managed to get most of the residents to safety, firefighters worked to beat back the quick-moving flames.
"They were on the second floor and trying to fight the fire, and there was a heavy volume of fire above them in the attic space. That is when they said, it is overcoming us; we have to get out and it will go to a defensive operation," said Jim Jackson, Downers Grove Fire Department.
Firefighters were called back to the scene Monday morning to put out hot spots the blaze left behind.
Residents say they lost everything in the fire.
"Yeah, it was just scary for me. I just lost everything, everything, completely gone. There is, like nothing there. We were just looking through my stuff that was there and I can't find anything," said resident Raul Romero.
Tenant Grace Krauss, a seamstress, said the destruction caused by the fire left her uncertain about the future.
"In one day you lose everything," said Krauss. "You don't know where to go."
Building resident Heather Pavlik said her children fortunately were not home at the time of the fire that destroyed her family's belongings.
"We have two daughters - thank god they were at Grandma's house last night - spur of the moment. God was watching over her," said Pavlik. "But that's everything - our whole life is gone."
Only half of the units in the building were rented at the time of the blaze. Officials say that it is fortunate that no one was injured.
Property manager Rob Gudin said that once he got to the scene, the fire was already visible from outside.
"[Residents] said they saw sparks coming out from inside the lobby, other people said no, it's coming up on the roof," said Gudin. "I wasn't here when it first started, but when I got here, you could see the flames starting to come up off the roof"
Many residents are staying in vacant units in the building next door, and the Red Cross is helping people find shelter if they don't have family or friends in the area. The Red Cross also provided food and blankets for displaced people at a nearby elementary school.